ε-Caprolactam is a precursor for the preparation of Nylon-6. Nylon-6 was first made in 1899 by heating 6-aminohexanoic acid. Commercially feasible synthesis from ε-caprolactam (CL) was discovered by Paul Schlack at I. G. Farbenindustrie in 1938. Currently, approximately 95 wt % of the world's ε-caprolactam is produced from cyclohexanone oxime via the Beckmann rearrangement. The starting material for cyclohexanone can be cyclohexane, phenol, or benzene. Then, through a series of reductions and/or oxidations, cyclohexanone is formed. The latter is then reacted with a hydroxylamine salt, usually the sulfate, to form the oxime and ammonium sulfate. The oxime is rearranged in concentrated sulfuiric acid, and the resulting lactam sulfate salt is neutralized with ammonia to form ε-caprolactam and more ammonium sulfate. Subsequently, pure ε-caprolactam is obtained by a number of separation and purification steps. Currently, this process is extremely capital intensive and generates large quantities of waste.
An economically attractive method of production of ε-caprolactam uses 6-aminocapronitrile (ACN). U.S. Pat. No. 2,301,964 (E. I. Du Pont de Nemours & Company) discloses the production of lactams from aminonitriles and water in a liquid-phase method. Hydrolysis and concurrent lactam formation proceed rapidly when aminonitrile is reacted in a weak aqueous solution. Temperatures of from 200° C. to 375° C. are employed. The aminonitrile and water are maintained at this reaction temperature for not more than 1 hour. The reaction is preferably catalyzed with hydrogen sulfide.
U.S. Pat. No. 2,357,484 (issued to Martin, E. I. Du Pont de Nemours & Company) discloses a vapor-phase catalytic process for preparing N-substituted amides. The process comprises passing a vaporized mixture of water and an aliphatic aminonitrile, containing at least one aminonitrile moiety, over a dehydration-type catalyst at a temperature of typically from 150° C. to 500° C. for not more than 1 minute. When an open-chain aliphatic aminonitrile is used, in which the amino and nitrile groups are separated by at least two carbon atoms in contiguous relation, the product obtained is a lactam.
In recent years, inexpensive adiponitrile (ADN) has been made by the direct hydrocyanation of butadiene. This has led to a renewed interest in the Martin CL process because inexpensive ADN can be partially hydrogenated and refined to produce an impure product that comprises ACN. This product may contain some byproducts of the hydrogenation reaction, notably tetrahydroazepine (THA).
U.S. Pat. No. 6,716,977 discloses a method for making CL from impure ACN containing THA, comprising the following steps:
(1) Contacting the impure ACN with water at elevated temperature in the presence of a dehydration catalyst, both the impure ACN and the water being in the vapor phase, to produce a vapor phase reaction product that comprises CL, ammonia, water, ACN, and THA;
(2) Separating the ammonia and a major portion of the water from the vapor phase reaction product to produce a crude liquid CL comprising CL, ACN and THA;
(3) Introducing the crude liquid CL into a low boiler distillation column and removing a major portion of both the THA and ACN as a low boiler column distillate, and removing CL, high boilers and at most a minor portion of both the THA and ACN as a low boiler column tails; and
(4) Introducing the low boiler column tails into a high boiler distillation column and removing CL and at most a minor portion of the high boilers as a high boiler column distillate product and removing a major portion of the high boilers as a high boiler column tails.
In this method, separation of ACN & THA from CL requires a considerable number of stages in the low boiler column due to the difficulty of separation. A large number of stages in this column will cause increased pressure drop and excessively high temperature in the base of the column. It would, therefore, be desirable to have a process to make CL from ACN in which the impurities in the crude caprolactam product were converted into species having a higher vapor pressure, which would require fewer distillation stages.